1. Field of the Invention
The field of the present invention is gear pumps used in various hydraulic machines, for example an automatic transmission of an automobile, for generating a hydraulic pressure.
2. Description of the Related Art
Conventionally, automatic transmissions have a planetary gear unit with a plurality of rotating elements and a plurality of frictional engagement elements, i.e., clutches and brakes, for respectively engaging and releasing the rotating elements. Automatic transmission control is provided by controlling rotation of the plurality of rotating elements of the planetary gear unit by selective engagement and disengagement of the frictional engagement elements.
Engagement and disengagement of the frictional engagement elements is controlled by hydraulic pressure which, in turn, is controlled in accordance with vehicle speed or an operating condition by a hydraulic control unit of the automatic transmission. The hydraulic pressure supplied to the hydraulic control unit is produced by an oil pump as described in, for example, JP-A-11-82644.
As disclosed in JP-A-11-82644, the oil pump is a gear pump which generates the hydraulic pressure by a pump gear driven by the engine of the vehicle. The gear pump supplies the hydraulic pressure to the hydraulic control unit as mentioned above, to the torque converter of the automatic transmission and to lubricating portions of the automatic transmission.
As shown in FIGS. 5(a) and 5(b), a gear pump 1 is an internal tooth gear pump in which the pump gearing 2 includes a pair of drive gears 3 having a predetermined number of outer teeth 3a and a driven gear 4 having a predetermined number of inner teeth 4a. The drive gears 3 and the driven gear 4 are arranged eccentric to each other in a gear chamber 11 formed in a casing 10. The casing 10 includes a pump body (O/P body) 8 and a pump cover (O/P cover) 9. Portions of the outer teeth 3a and the inner teeth 4a mesh with each other at a boundary portion 7 of an intake port 5 and a delivery port 6. The tip of an outer tooth 3a1 and tip of an inner tooth 4a1 on a side substantially opposed to the intermeshed teeth, relative to the rotational center, are brought into contact with each other.
Further, by rotating the drive gears 3 via shaft 12 driven by the engine, not illustrated, and also rotating the driven gear 4 in the same direction by rotation of the drive gears 3, the outer tooth 3a and the inner tooth 4a which are brought into contact with each other, are successively changed. Further, by gradually increasing volume of a pump chamber 13 formed between an outer periphery of the drive gears 3 and an inner periphery of the driven gear 4 and communicating with the intake port 5, an operating fluid is drawn in through the intake port 5, the pump chamber 13 is cut off from the intake port 5 and communicates with the delivery port 6 and by gradually reducing the volume of the pump chamber 13 thereafter, the operating fluid is discharged from the pump chamber 13 through the delivery port 6.
In the conventional gear pump 1, as shown in FIG. 5(a), the total area of both side faces 3b and 3c of the drive gear 3 and both side faces 4b and 4c of the driven gear 4, contact and slide on a gear chamber side face 8a of the pump body 8 and a gear chamber side face 9a of the pump cover 9. Therefore, it is difficult to form oil films between the side faces 3b and 3c of the drive gear 3 and the side faces 4b and 4c of the driven gear 4 and the gear chamber side face 8a of the pump body 8 and the gear chamber side face 9a of the pump cover 9.
When the side faces 3b and 3c, 4b and 4c of the respective gears 3 and 4 and the gear chamber side faces 8a and 9a are brought into direct contact with each other in this way, wear is produced at the respective areas of contact and the delivery capacity of the gear pump 1 is thereby gradually reduced by that wear. Further, the sliding resistance also causes loss of torque in the gear pump 1.
Further, with an automatic transmission using the conventional gear pump 1, the driving force of the engine must be increased by the amount of loss of torque and therefore causes fuel consumption to increase.
Accordingly, an object of the present invention is to provide a gear pump facilitating formation of an oil film between side faces of a pump gear and a side face of a pump casing to thereby reduce wear at the respective areas of sliding contact and to thereby prevent a reduction in delivery and reduce loss of torque by reduction of sliding resistance.
It is another object of the present invention to provide an automatic transmission reducing the force required of a drive source to thereby reduce fuel consumption.
In a first aspect, the present invention provides a gear pump including a casing having an intake port, a gear chamber and a delivery port, and pump gearing including a pair of gears arranged in the gear chamber and in mesh with each other, whereby oil is taken in through the intake port and delivered from the delivery port. A recess is formed at at least one side face selected from the side faces of the gears and the side faces of the gear chamber of the casing on which the side faces of the gears slide.
According to a second aspect of the invention, one gear of the pair of gears is a drive gear having outer teeth and other gear of the pair of gears is a driven gear eccentric to the one gear and having inner teeth in mesh with the outer teeth.
According to a third aspect of the invention, when the recesses are formed in at least one side face of the drive gear and the side faces of the gear chamber of the casing on which the drive gear slides, the recesses are arranged on at least one of an inner circle disposed concentric with the drive gear and on an inner side of bottoms of spaces between the outer teeth and an outer circle disposed on an outer side of the bottoms. When the recesses are formed in at least one side face selected from the side faces of the driven gear and the side faces of the gear chamber of the casing on which the side faces of the driven gear slide, the recesses are arranged on a circle concentric with the driven gear and having a diameter the same or substantially the same as the diameter of bottoms of the spaces between the inner teeth.
According to a fourth aspect of the invention, when the recesses are arranged on the inner circle they lie on a diameter line passing through a center of the bottom of a space between the teeth. When the recesses are arranged on the outer circle they are located at the peripheral (circumferential) center of one of the outer teeth, and when the recesses are arranged on the circle having the same diameter or substantially the same diameter concentric with the driven gear, they are located at the peripheral (circumferential) center of one of the inner teeth.
In a fifth aspect, the present invention provides a gear pump having a casing formed with an intake port, a gear chamber and a delivery port, and a pair of gears rotatably mounted in the gear chamber of the casing and in mesh with each other. Oil is sucked in at the intake port and discharged from the delivery port by rotation of the pair of gears while the radially extending side surfaces of the gears slide on side faces of the gear chamber of the casing. At least one of the side faces of the pair of gears and the side faces of the gear chamber of the casing on which the gear side faces slide, has means for forming an oil film between the opposed sliding surfaces.
Further, according to a sixth aspect of the present invention, the means for forming the oil film of the fifth aspect comprises recesses for holding the oil.
A seventh aspect of the present invention provides an automatic transmission comprising a planetary gear unit having a plurality of rotating elements, a plurality of frictional engagement elements including at least a clutch and a brake for selectively engaging and releasing the plurality of rotating elements, and a hydraulic control unit for supplying a controlled hydraulic pressure, produced by controlling the hydraulic pressure supplied from an oil pump, at a predetermined magnitude to the frictional engagement elements. Rotation of a drive source, e.g., an engine, is input to the automatic transmission which changes its speed by hydraulically controlling engagement and disengagement of the frictional engagement elements utilizing the hydraulic control unit. The oil pump is a gear pump according to any one of the first through fourth aspects of the invention.
In the gear pump according to the first through the fourth aspects of the invention, the recesses are formed in at least one of six side faces which include the side faces of the pair of gears and the side faces of the gear chamber of the casing on which the respective side faces of the gears slide, which recesses store oil. The oil stored in the recesses oozes out between the opposing and contacting side faces due to centrifugal force produced by rotation of the gears and due to capillary action to thereby form oil films therebetween.
With the gear pump-of the first through the sixth aspects of the invention, oil films are formed between the side faces of the gears and the side faces of the gear chamber of the casing to thereby reduce wear of the sliding faces of the pair of gears and of the gear chamber and to thereby prevent change of the delivery rate with aging of the gear pump. Further, the oil films formed between the sliding side faces reduce sliding resistance therebetween and loss of torque.
Particularly, according to the first through the fourth aspects of the invention, the recesses can be formed by a sintered mold simultaneously with molding of the gears or the casing and, therefore, working steps and cost can be made substantially equivalent to manufacture of the conventional gear pump. Further, by providing the recesses, the material of the gears and/or the casing can be saved and, therefore, the weight of the gears and/or the casing can be reduced and material cost can be saved. Thereby, cost of the gear pump is also reduced.
Further, according to the first through the fourth aspects of the invention, the recesses are provided in the pump gears and/or the casing of an otherwise conventional gear pump and, therefore, a conventional pump gear or casing can be used and an entirely new pump gear need not be formed. Therefore, considerable design change is not needed and cost can be further reduced.
Particularly according to the third aspect of the invention, by providing the drive gear with the recesses arranged on an outer circle concentric with the drive gear and the recesses arranged in the driven gear, the recesses can consecutively face the intake port and the delivery port by rotation of the drive and driven gears and, therefore, the oil can be effectively stored in the recesses facing the ports. Thereby, the oil films can be reliably formed between (1) the side faces of the drive gear(s) and/or the side faces of the driven gear and (2) the side faces of the gear chamber of the casing.
Further, according to the fifth and the sixth aspects of the invention, the means for forming the oil film includes projections, it is not necessary to provide the recesses at both the pair of gears and the casing. Particularly, according to the sixth aspect of the invention, by forming either the side faces of the pair of gears or the side faces of the gear chamber of the casing with the recesses and projections as the means for forming the oil film, working of the other of the pair of gears or the casing can be avoided.
Further, according to the automatic transmission of the seventh aspect of the invention, the gear pump is an otherwise conventional gear pump but, as mentioned above, loss of torque can be reduced and, therefore, drive force of the engine can be reduced by that amount, fuel cost can be improved and energy conservation and a reduction in emissions can be achieved. Further, since the drive force of the engine can be reduced, the engine can be made smaller and, given the additional weight reduction of the drive gears, the drivetrain of the vehicle can be made smaller in size and more compact.